Electroless deposition of palladium in the manufacture of printed circuit boards, IC substrates and the like as well as metallisation of semiconductor wafers is an established technique. The palladium layers are used for example as barrier layers and/or wire-bondable and solderable finishes.
Electroless palladium plating bath compositions comprising a source for palladium ions, a nitrogenated complexing agent and a reducing agent selected from formic acid and derivatives thereof are disclosed in U.S. Pat. No. 5,882,736. Such electroless palladium plating bath compositions are suited to deposit pure palladium in contrast to plating bath compositions containing hypophosphite as reducing agent which result in palladium-phosphorous alloy layers.
The stability of plating bath compositions comprising palladium ions is an important feature of such plating bath compositions due to the high price of palladium and the requirement for palladium layers deposited having predictable properties such as internal stress, and a high adhesion to the underlying substrate whereon the palladium layer is deposited.
The stability of such a plating bath means that the plating bath is stable against decomposition, i.e. the undesired precipitation of metallic palladium in the plating bath itself. Accordingly, a stable plating bath has a longer life time than an unstable plating bath. At the same time, the deposition rate of palladium from such a plating bath should be high enough, for example more than 10 nm/min, in order to fulfil the requirements for an industrial palladium plating method.